Dies when in drive not in reverse?
My first suspect would be the gear selector switch, this switch tell the ECM what gear the vehicle is in, if no signal for drive then the ECM still believes it is in park or neutral so the idle is not increased or the timing adjusted or the air mixture adjusted, all of this happens in milliseconds so you won't fell any of it happening. A good scan tool with live data will tell you what is going on.
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